The present invention relates to a vulcanizable rubber composition which can be used for the manufacture of tires and a process for the preparation of this composition. The invention also provides a tire, the tread of which comprises such a composition. The rubber composition of the invention possesses improved hysteresis and physical properties in the vulcanized state, while retaining satisfactory processing properties in the non-vulcanized state.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce rubber mixes having good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, sidewalls or treads, and in order to obtain tires having reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, in particular those that modify the structure of diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or starring agents. The very great majority of these solutions have concentrated on the use of functionalized polymers which are active with respect to carbon black, with the aim of obtaining a good interaction between the polymer, thus modified, and the carbon black.
By way of illustration, the prior art relating to reinforcing fillers formed of carbon black, includes, for example, U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the end of a chain with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as coupling or starring agent for diene polymers.
Silica has long been used as a reinforcing filler in vulcanizable rubber compositions, in particular those intended for use in tire treads. However, this use has remained very limited, because of an unsatisfactory level of certain physical properties of such compositions, in particular abrasion resistance.
In order to overcome these drawbacks, the use functionalized diene polymers instead of the non-functionalized polymers which were used before has been proposed. In particular, polymers functionalized by alkoxysilane derivatives, such as tetra-ethoxysilanes, has been proposed. For example, U.S. Pat. No. 5,066,721 describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolyzable alkoxyl radical, which makes it possible to eliminate the polymerization solvent by steam stripping.
One disadvantage of these functionalization reactions relates to the coupling reactions which accompany them, which generally makes it necessary to use an excess of alkoxysilane and/or intensive mixing, in order to minimize these coupling reactions.
Another drawback of these reactions involves the later implementation of the steam stripping operation, which is necessary to eliminate the polymerization solvent.
Generally, experience has shown that the functionalized polymers obtained in this fashion undergo changes in macrostructure during the stripping operation, which results in serious degradation of their properties, unless the functionalizing agent is an alkoxysilane belonging to a restricted family, such as that described in U.S. Pat. No. 5,066,721.
Consequently, the use of diene polymers comprising an alkoxysilane function to produce rubber compositions comprising silica as reinforcing filler has not proven satisfactory, despite the improved physical properties of these compositions.
This is why further research has been carried out on other functionalization reactions with the view to obtaining such rubber compositions.
For example, French patent specification FR-A-2 740 778 of the assignee of the present inventors discloses the use of diene polymers bearing a silanol function or a polysiloxane block having a silanol end at the chain end. For example, a cyclic polysiloxane functionalizing agent is used, such as hexamethylcyclotrisiloxane. The functionalized polymers obtained can be separated from the reaction medium resulting in their formation by steam extraction of the solvent, without their macrostructure and, consequently, their physical properties changing.
These functionalized polymers are intended to be incorporated in vulcanized compositions comprising silica as the majority reinforcing filler, including fillers comprising a blend of silica and carbon black. It has been possible to establish that these polymers impart rubber properties, in particular hysteresis and reinforcement properties in the vulcanized state, which are improved compared with those of control compositions based on non-functionalized diene polymers. Such compositions are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane function.
However, this improvement of the hysteresis and reinforcement properties in the vulcanized state is accompanied by an increase in the Mooney viscosity of the compositions tested relative to control compositions, i.e., by a reduced ability to be worked in the non-vulcanized state.
The object of the present invention is to overcome the above-identified limitations of the prior art. The inventors have surprisingly discovered that a rubber composition comprising a reinforcing white filler and at least one diene block copolymer which is intended to interact with said reinforcing white filler, wherein said copolymer comprises on at least one end thereof a polysiloxane block ending in a trialkylsilyl group said polysiloxane block corresponding to the formula:
[xe2x80x94(SiR1R2O)xe2x80x94]xxe2x80x94SiR3R4R5, 
in which R1, R2, R3, R4 and R5 each represent alkyl groups having from 1 to 20
carbon atoms and in which x is a natural integer other than zero,
results in hysteresis properties at small and large deformations which are comparable to those achieved with the functionalized diene polymers mentioned above. In addition, the rubber composition results in processing properties in the non-vulcanized state which are particularly advantageous, because they are close to those obtained with a composition based on a non-functionalized diene polymer.
Preferably, the molecular weight of the polysiloxane block is between 500 and 5,000 g/mol.
The diene block copolymer of the invention is produced by reacting a diene polymer with a functionalized polysiloxane. It will be noted that this reaction can be performed continuously or discontinuously.
This diene block copolymer may be produced in two different ways:
(1) Reacting a living diene polymer with a polysiloxane block having an Sixe2x80x94Cl function at the chain end. More precisely, another polymer comprising said polysiloxane is grafted onto this living polymer, this other polymer being obtained anionically by means of an initiator comprising an alkyl group as carbanion. This method advantageously allows for the possibility of introducing into the reaction medium a reduced amount of a polar solvent.
(2) Carrying out sequential polymerization, wherein one or more diene monomers is/are polymerized to produce a living diene polymer block, the living diene polymer is then being reacted in a polar solvent with a cyclic siloxane which is polymerized anionically, to form a second block. At the end of the polymerization, the copolymer thus obtained is functionalized by a trialkylchlorosilane.
Published French patent specification FR-A-2 740 778 of the assignee of the present invention describes such sequential polymerization, wherein the first and second blocks are, for example, obtained respectively by copolymerization of butadiene and styrene, and by polymerization of hexamethylcyclotrisiloxane.
In regard to the present invention, xe2x80x9cdiene polymerxe2x80x9d is understood to mean any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms, or any copolymer obtained by copolymerization of one or more dienes conjugated together or with one or more vinyl aromatic compounds having 8 to 20 carbon atoms. Such copolymers contain 20% to 99% by weight of diene units and 1 to 80% by weight of vinyl aromatic units.
Suitable conjugated dienes include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1 to C5 alkyl)-1,3-butadienes, such as 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, and phenyl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Suitable vinyl aromatic compounds include, styrene, ortho-, para- or meta-methylstyrene, the commercial xe2x80x9cvinyl-toluenexe2x80x9d mixture, para-tert.butylstyrene, the methoxy-styrenes, vinyl mesitylene, divinyl benzene and vinyl naphthalene.
The following diene polymers/copolymers are preferred for the present invention:
polybutadienes, in particular those having a mass content of 1,2-units from 4% to 80%,
polyisoprenes having a content of vinyl units of less than 80%,
butadiene-styrene copolymers, in particular, those having a mass content of styrene from 4% to 50%, more particularly from 20% to 40%, and mass contents of 1,2-bonds and trans-1,4 bonds from 4% to 65% and 30% to 80% respectively,
butadiene-isoprene copolymers, in particular those having a mass content of isoprene from 5% to 90% and a glass transition temperature (Tg) ranging from xe2x88x9280xc2x0 C. to xe2x88x9220xc2x0 C.,
isoprene-styrene copolymers, in particular those having a mass content of styrene from 5% to 50% and an amount of vinyl units which is less than 90%,
butadiene-styrene-isoprene terpolymers, in particular those having a mass content of styrene from 5% to 50%, more particularly from 10% to 40%, a mass content of isoprene from 15% to 60%, more particularly from 20% to 50%, a mass content of butadiene from 5% to 50%, more particularly from 20% to 40%, a mass content of 1,2 units of the butadiene part from 4% to 85%, a mass content of trans-1,4 units of the butadiene part from 6% to 80%, a mass content of 1,2- and 3,4-units of the isoprene part from 5% to 70%, and a mass content of trans-1,4 units of the isoprene part from 10% to 50%.
Preferably, the diene polymer is obtained anionically, by means of a monofunctional or bifunctional initiator, depending on whether it is desired to obtain, for said block copolymer, a polysiloxane block ending in a trialkylsilyl group on only one or on both of the two chain ends, respectively. This initiator may be an alkyllithium, preferably n-butyllithium, or a lithium amide.
The functionalized polysiloxane bears a halo-organosilane function at the chain end and a trialkylsilyl function at the other end. It is obtained by first polymerizing a cyclic siloxane, which is initiated by an organometallic compound having a carbon-metal bond, preferably an organolithium compound, to form a polysiloxane and then functionalizing the polysiloxane by means of a dihalo-organosilane.
Preferably, hexamethylcyclotrisiloxane is used as the cyclic siloxane, n-butyllithium is used as initiator and dichlorodimethylsilane is used as the functionalizing agent to produce the functionalized polysiloxane. Thus, each polysiloxane block would be a polydimethylsiloxane (abbreviated to PDMS in the rest of the present description), which ends in a trialkylsilyl group.
Alternatively, the functionalized polysiloxane may be prepared using sec.-butyllithium, n-propyllithium or ethyllithium as initiator, in order to obtain a polysiloxane, the end trialkylsilyl group of which comprises a sec.-butyl, n-propyl or ethyl radical, respectively.
A rubber composition according to the invention is obtained by mixing, by means of thermomechanical working, said diene block copolymer comprising at least one polysiloxane block which ends in a trialkylsilyl group with a reinforcing white filler present in a majority portion in the reinforcing filler used and with additives suitable for obtaining a vulcanizable rubber composition.
According to variant embodiments of the composition according to the invention, said diene block copolymer may be used in a blend with one or more other elastomers conventionally used in tires, such as natural rubber or a blend based on natural rubber and a synthetic elastomer, or alternatively another diene elastomer which may be coupled and/or starred, or alternatively partially or completely functionalized by a block other than a polysiloxane block which ends in a trialkylsilyl group.
However, the improvement in the properties of the rubber composition according to the invention are greater, when a lower proportion of such conventional elastomers are used. Advantageously, such conventional elastomers may be present in a quantity of from 1 to 70 parts by weight per 100 parts by weight of the diene block copolymer comprising at least one polysiloxane block ending in a trialkylsilyl group according to the invention.
xe2x80x9cReinforcing white fillerxe2x80x9d is understood herein to mean a xe2x80x9cwhitexe2x80x9d filler (i.e. an inorganic filler, particularly a mineral filler), sometimes also called xe2x80x9cclearxe2x80x9d filler, which is capable, on its own, without any means other than an intermediate coupling system, of reinforcing a rubber composition intended for the manufacture of tires. In other words the reinforcing white filler is capable of replacing a conventional filler of tire-grade carbon black in its reinforcement function.
Preferably, the reinforcing white filler is, in its entirety or at least in a majority portion, silica (SiO2). The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or pyrogenic silica having a BET surface area and a specific CTAB surface area both of which are less than 450 m2/g. The highly dispersible precipitated silicas are preferred.
In the present specification, the BET specific surface area is determined in known manner, in accordance with the method of Brunauer, Emmet and Teller described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, vol. 60, page 309, February 1938, and corresponding to Standard AFNOR-NFT-45007 (November 1987); the CTAB specific surface area is the external surface area determined in accordance with the same Standard AFNOR-NFT-45007 of November 1987.
xe2x80x9cHighly dispersible silicaxe2x80x9d is understood to mean any silica having a very substantial ability to disagglomerate and to disperse in an elastomeric matrix, which can be observed in known manner by electron or optical microscopy on thin sections. Non-limiting examples of such preferred highly dispersible silicas include the silica Perkasil KS 430 from Akzo, the silica BV 3380 from Degussa, the silicas Zeosil 1165 MP and 1115 MP from Rhodia, the silica Hi-Sil 2000 from PPG, the silicas Zeopol 8741 or 8745 from Huber, and treated precipitated silicas, such as the aluminium-xe2x80x9cdopedxe2x80x9d silicas described in application EP-A-0 735 088.
The physical state in which the reinforcing white filler is present is immaterial; thus it may be in the form of a powder, microbeads, granules or balls. xe2x80x9cReinforcing white fillerxe2x80x9d is also understood to mean mixtures of different reinforcing white fillers, in particular, mixtures of highly dispersible silicas such as described above.
According to a preferred example of embodiment of the invention, the reinforcing filler of the rubber composition comprises a majority of reinforcing white filler (i.e., the mass fraction of reinforcing white filler in the entire reinforcing filler of the rubber composition is greater than 50%).
The reinforcing filler also may be a blend (mixture) of fillers, comprising the above-mentioned reinforcing white filler(s) and carbon black in a minority proportion (i.e., in a mass fraction of less than 50% of the total amount of reinforcing filler). Suitable carbon blacks are all the carbon blacks, in particular the blacks of the type HAF, ISAF and SAF, which are conventionally used in tires, and particularly in treads for tires. Non-limiting examples of such blacks, include the blacks N115, N134, N234, N339, N34 7 and N375.
For example, black/silica blends or blacks partially or integrally covered with silica are suitable to form the reinforcing filler. Also suitable are carbon blacks modified by silica, including, inter alia, the fillers sold by CABOT under the name xe2x80x9cCRX 2000xe2x80x9d, which are described in WO-A-96/37547.
Where the reinforcing filler contains only a reinforcing white filler and carbon black, the mass fraction of the carbon black in said reinforcing filler is preferably selected to be less than or equal to 30% of the amount of reinforcing filler.
However, experience has shown that the aforementioned properties of the composition according to the invention are most greatly involved, the higher the mass fraction of reinforcing white filler that is used in the reinforcing filler in the rubber composition. Said properties are optimum when said rubber composition contains solely a reinforcing white filler, for example silica, as reinforcing filler. The latter, therefore, constitutes a preferred example of a rubber composition according to the invention.
The rubber composition according to the invention furthermore comprises, in conventional manner, a reinforcing white filler/elastomeric matrix bonding agent (also referred to as coupling agent), the function of which is to ensure sufficient chemical and/or physical bonding (or coupling), between the white reinforcing filler and the elastomeric matrix, while facilitating the dispersion of the white filler within said matrix.
Such a bonding agent, which is at least bifunctional, has, for example, the simplified general formula xe2x80x9cYxe2x80x94Txe2x80x94Xxe2x80x9d, in which:
Y represents a functional group (xe2x80x9cYxe2x80x9d function) which is capable of bonding physically and/or chemically with the white filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the hydroxyl (OH) surface groups of the filler (for example, surface silanols in the case of silica);
X represents a functional group (xe2x80x9cXxe2x80x9d function) which is capable of bonding physically and/or chemically with the elastomer, for example by means of a sulphur atom; and
T represents a hydrocarbon group that links Y and X.
These bonding agents are not to be confused with simple agents for covering the filler in question which, in known manner, may comprise the Y function which is active with respect to the filler but are devoid of the X function which is active with respect to the elastomer.
Such bonding agents, of variable effectiveness, have been described in a large number of documents and are well-known to those skilled in the art. In fact, any known bonding agent may be used to ensure effective bonding between the silica and diene elastomer in diene rubber compositions which can be used for the manufacture of tires. Such bonding agents include organosilanes, in particular polysulphurized alkoxysilane or mercaptosilanes, or polyorganosiloxanes bearing the X and Y functions mentioned above.
In particular, polysulphurized alkoxysilanes are used, such as those described in U.S. Pat. Nos. 3,842,111, 3,873,489, 3,978,103, 3,997,581, 4,002,594, 5,580,919, 5,583,245, 5,663,396, 5,684,171, 5,684,172 and 5,696,197.
Particularly suitable bonding agents for the rubber composition of the invention, without the definition below being limiting, are so-called xe2x80x9csymmetricalxe2x80x9d polysulphurized alkoxysilanes which satisfy the following general formula (I):
Zxe2x80x94Axe2x80x94Snxe2x80x94Axe2x80x94Z,xe2x80x83xe2x80x83(I) 
in which:
n is an integer from 2 to 8;
A is a divalent hydrocarbon radical;
Z corresponds to one of the following formulae: 
in which:
R1, which may or may not be substituted, and may be identical or different, each represents a C1-C18 alkyl group, a C5-C18 cycloalkyl group, or a C6-C18 aryl group;
R2, which may or may not be substituted, and may be identical or different, each represents a C1-C18 alkoxyl group or a C5-C18 cycloalkoxyl group.
In formula (I) the number n is preferably an integer from 3 to 5.
In the case of a mixture of polysulphurized alkoxysilanes in accordance with Formula (I) particularly conventional, commercially available, mixes, the average value of xe2x80x9cnxe2x80x9d is a fractional number, preferably between 3 and 5, more preferably close to 4.
Radical A, whether substituted or not, is preferably a divalent, saturated or non-saturated hydrocarbon radical, comprising 1 to 18 carbon atoms. In particular A comprises a C1-C18 alkylene group or C6-C12 arylene group, more particularly a C1-C10 alkylene, in particular a C2-C4 alkylene, preferably propylene.
The R radicals are preferably a C1-C6 alkyl, a cyclohexyl or a phenyl, in particular a C1-C4 alkyl group, more particularly methyl and/or ethyl.
The R1 radicals are preferably a C1-C8 alkoxyl or a C5-C8 cycloalkoxyl, more particularly methoxyl and/or ethoxyl.
Such so-called xe2x80x9csymmetricalxe2x80x9d polysulphurized alkoxysilanes, and processes for obtaining them are described, for example, in U.S. Pat. Nos. 5,684,171 and 5,684,172, which give a detailed list of these known compounds, for n varying from 2 to 8.
Preferably, the polysulphurized alkoxysilane used in the invention is a polysulphide, in particular a tetrasulphide, of bis(alkoxy(C1-C4)silylpropyl), more preferably of bis(trialkoxy(C1-C4)silylpropyl), in particular of bis(3-triethoxysilylpropyl) or of bis(3-trimethoxysilylpropyl).
A particularly preferred bonding agent for use in the present invention is bis(triethoxysilylpropyl) tetrasulphide, or TESPT, of the formula [(C2H5O)3Si(CH2)3S2]2, which is sold by Degussa under the name Si69 (or X50S when it is supported to 50% by weight on carbon black), or by Witco under the name Silquest A1289. In both cases, the bonding agent is a commercial mixture of polysulphides having an average value of n which is close to 4.
Those skilled in the art will be able to adjust the content of bonding agent in the compositions of the invention, according to the intended application, the elastomeric matrix used and the quantity of reinforcing white filler used.
In the rubber compositions according to the invention, the content of bonding agent may be within a range from 0.5 to 15% relative to the mass of reinforcing white filler, preferably, within a range from 5 to 10%.
The bonding agent may be grafted beforehand (via the xe2x80x9cXxe2x80x9d function) onto the diene elastomer of the composition of the invention, the elastomer thus functionalized or xe2x80x9cprecoupledxe2x80x9d thereby comprising the free xe2x80x9cYxe2x80x9d function for the reinforcing white filler. It could also be grafted beforehand (via the xe2x80x9cYxe2x80x9d function) onto the reinforcing white filler, the thus xe2x80x9cprecoupledxe2x80x9d filler then being able to be bonded to the diene elastomer by means of the free xe2x80x9cXxe2x80x9d function.
It is preferred, however, for reasons of better processing of the compositions in the non-vulcanized state, to use the bonding agent, either grafted onto the reinforcing white filler, or in the free (i.e. non-grafted) state.
Additional additives which are used in this composition, include plasticizers, pigments, antioxidants, antiozone waxes, a vulcanization system based either on sulphur and/or on peroxide and/or on bismaleimides, vulcanization accelerators, extender oils, one or more agents for covering the silica, such as alkoxysilanes, polyols or amines.
A further subject of the invention is a tread for a tire, which comprises a rubber composition according to the invention.
Because of the reduced hysteresis which characterizes the rubber composition according to the invention in the vulcanized state, it will be noted that a tire, the tread of which comprises said composition has an advantageously reduced rolling resistance.
The aforementioned characteristics of the present invention, as well as others, will be better understood on reading the following description of several examples of embodiment of the invention, which are given by way of illustration and not of limitation.
In the examples which follow, the viscosities are inherent viscosities which are measured in toluene, at a concentration of 1 g/l.